The present invention relates generally to an exposure apparatus, and more particularly to a measurement of a polarization state of an exposure light and projection optical system used for the exposure apparatus that exposes a substrate, such as a single crystal substrate for a semiconductor, and a glass plate for a liquid crystal display.
A conventional projection exposure apparatus uses a projection optical system to project and transfer a circuit pattern of a reticle (or a mask) onto a wafer, etc., in manufacturing a semiconductor device in a photolithography technology.
The projection exposure apparatus has three important factors, resolution, overlay accuracy, and throughput, as a factor that determines an exposure performance. Recently, particularly concerning to the resolution among these three factors, a higher numerical aperture (NA) of the projection optical system by an immersion exposure attracted people's attentions. The higher NA of the projection optical system, which is referred to as a high NA imaging, means an increase of an angle between a perpendicular to an image surface and a traveling direction of an incident light.
A polarization state of the exposure light is important in the high NA imaging. For example, assume exposure of a so-called line and space (L & S) pattern that has repetitive lines and spaces. The L & S pattern is formed by a plane wave two light interference. An incident plane is defined as a plane including incident vectors of two lights, a S-polarized light is a polarized light perpendicular to the incident plane, and a P-polarized light is a polarized light parallel to the incident plane. When an angle is 90 degrees between the incident vectors of two lights, the two S-polarized lights interfere with each other, forming a light intensity distribution corresponding to the L & S pattern on the image surface. On the other hand, the two P-polarized lights neither interfere with and cancel with each other, nor form the light intensity distribution corresponding to the L & S pattern on the image surface. The light intensity distribution with a blend of the S-polarized light and the P-polarized light has a lower contrast on the image surface than that with only the S-polarized light. As a ratio of the P-polarized light increases, the contrast of the light intensity distribution lowers on the image surface, and the pattern is not formed at last.
Thus, it is necessary to control the polarization of the exposure light and to improve the contrast. The polarization-controlled exposure light forms a light intensity distribution having a sufficient contrast on the image surface, and realizes a fine pattern resolution.
An illumination optical system, more specifically, a pupil in the illumination optical system controls a polarization state of the exposure light. A polarization illumination requires illuminating using an effective light source having an effective form to a certain pattern and an optimal polarization direction. For example, a X dipole illumination having the polarization direction in a Y direction is effective to a Y direction pattern. Moreover, a tangential illumination having the polarization direction in a circumferential direction of an annular is effective to a pattern that includes patterns in various directions.
However, even when the pupil in the illumination optical system controls the polarization of the exposure light, the polarization state controlled at the pupil in the illumination optical system is not always maintained up to the image surface due to the influences of the optical system subsequent to the pupil in the illumination optical system and/or the projection optical system. For example, although a lens or a reflecting mirror has an antireflection coating or a high reflection coating to improve the transmittance or the reflectance, a reflectance is different according to the polarization direction and an element that changes the polarization by applying a phase difference exists. A crystal material, such as quartz and fluorite, is used as glass materials according to a shorter wavelength of the exposure light. These glass materials include a birefringence and change the polarization state. Moreover, the birefringence included in the glass materials changes by a stress of a mechanical member, such as a lens barrel, that maintains the glass materials. Therefore, it is very difficult to always maintain the birefringence of the glass materials to a constant.
Thus, it is necessary to measure a polarization state of the exposure apparatus, in other words, a polarization state of the projection optical system. For example, the exposure apparatus that includes a measuring means on a wafer stage and can measure the polarization state at a wafer side in the projection optical system. See, for example, Japanese Patent Application, Publication No. 2004-61515.
On the other hand, in a higher resolution and a critical dimension control, the importance for measurement and control of an effective light source distribution of the illumination optical system is also recognized. The effective light source distribution of the illumination optical system means, for example, a size, form and light intensity distribution of a light source image on an entrance pupil plane to a size of the entrance pupil plane in the projection optical system in the projection exposure apparatus that uses a Koehler illumination. In measurement of this effective light source form, the measuring means is provided on the wafer stage, and the effective light source form is measured via the projection optical system.
Thus, in the recently projection exposure apparatus, the highly accuracy measurement of the polarization state on the exposure apparatus is a necessary matter.
However, prior art that provides the measuring means on the wafer stage includes various problems, when applying to the recently projection exposure apparatus.
The most serious problem is as follows. The influence on the polarization state on the exposure apparatus by each factor of the illumination optical system and the projection optical system cannot be separated and measured. In the prior art, the polarization state of the illumination optical system and the projection optical system (hereinafter, referred to as an “exposure optical system”) on the wafer surface is usually measured without arranging the reticle (in a state of only 0th light). In such measurement, since the polarization state of the illumination optical system and the polarization state of the projection optical system are cancelled, a seemingly good polarization state may be measured. However, in actual pattern transfer, the reticle is arranged between the illumination optical system and the projection optical system. Therefore, if the circuit pattern on the reticle is illuminated, not only the 0th light that transmits the reticle but high order diffracted lights according the circuit pattern occurs. Each of such 0th light and high order diffracted lights passes different optical paths in the projection optical system, condenses on the wafer and forms the pattern image. Then, even if the polarization state is good at the measurement of the polarization state on the wafer surface, the polarization state of each high order diffracted light on the optical path is different. As a result, the imaging performance becomes asymmetry, and defective resolution is caused.
Moreover, an optical member to measure the polarization with high accuracy cannot be arranged by a restriction of space on the wafer stage. For example, on a periphery of a lens, an internal stress by a maintenance occurs and the birefringence of the glass materials is changed. Therefore, a measurement error by change of the polarization state occurred in the measuring means is included in a measurement value.
In addition, a polarization element and a phase shifter cannot be arranged in the measuring means by the restriction of space on the wafer stage. Then, the polarization state of the projection optical system is obtained by measuring the illumination light, which has been polarization-controlled by the illumination optical system, by the measuring means that does not have a function of polarization measurement on the wafer stage. Therefore, the highly accuracy polarization measurement including a phase change cannot be executed.
The measuring means also needs the higher NA corresponding to the higher NA by the immersion exposure. However, it is difficult to provide the measuring means with high NA on the wafer stage.